Leads or catheters having pressure sensors are used in a number of different implantable medical devices. For example, the Implantable Hemodynamic Monitor (IHM) made by Medtronic, Inc., assignee of this invention, continuously monitors right ventricular pressure. Pacemaker systems likewise employ pressure sensors and a number of other applications currently use pressure, or potentially can use pressure sensors for obtaining significant information.
Pacemaker leads having a pressure sensor are used in connection with cardiac pacemakers, and particularly in rate response pacemakers, e.g., VVIR, DDDR, etc. Pressure sensors may likewise be utilized on pacing leads for other purposes, such as determining capture by a delivered pace pulse. The pressure sensor on the pacemaker lead detects the pressure inside the heart chamber into which the lead is inserted, and converts the detected pressure into electronic information which is transmitted back to the pacemaker. The pacemaker then analyzes the information received from the pressure sensor and determines an appropriate rate response or uses the information for another purpose. Pacemaker systems utilizing, e.g., ventricular pressure, for determining an appropriate rate response are well known in the art. See U.S. Pat. Nos. 4,052,991 and 4,600,017; EPO Pub. No. 0 017 528.
Prior to implanting a pacemaker lead having a pressure sensor into a patient, it is critical to establish the accuracy of the pressure sensor. The accuracy of the pressure sensor can be divided into two parameters, "baseline pressure" and "scale factor." In regard to the former parameter, the pressure detected by the sensor must accurately reflect the true pressure inside the particular chamber into which it is inserted. Thus, the "baseline pressure" value of the sensor must be established relative to a known applied pressure. In regard to the latter parameter, any increase or decrease in pressure reported by the sensor in the chamber of the heart into which it has been inserted must accurately reflect the amount of pressure actually increased or decreased. Thus, an accurate "scale factor" for the sensor must be established, in order to have complete calibration. In addition, it is preferable that such parameters be adjusted within a sterile environment.
The accuracy of the pressure sensor is not guaranteed, or at least should not be relied upon by physicians, simply upon the manufacturer's packaging of the pacemaker lead. Once implanted into a patient, the pressure sensor accuracy cannot be easily verified. It has long been desired to provide reliable calibration of baseline and scale factor for a pressure sensor in a pacemaker lead, and to do so in a sterile environment. To this end, Applicant's invention is directed to apparatus and methods of using the apparatus to establish an accurate baseline and scale factor for a pressure sensor in a pacemaker lead in a sterile environment. In patients with cardiac conditions which require implantation of such a pacemaker lead, Applicant's invention provides easy but reliable calibration, to insure that accurate data is detected and transmitted to the pacemaker. Applicant's invention also provides the physician with confidence that sufficient care has been taken to accurately calibrate the pressure sensor of the pacemaker lead in a sterile environment. The use of the invention thus minimizes any need to replace or reprogram the implanted pacemaker to account for an inaccurate pressure sensor, much to the benefit of the patient.
Although the description of this invention uses an implantable pacemaker system as exemplary, it is to be noted that it applies equally to other uses and applications. In addition to the IHM described above, it is applicable to any system where pressure is sensed, e.g., for detecting tachycardia or fibrillation, detecting heart failure, etc.